The Long Term Evolution (LTE) system of the 3rd Generation Partnership Project (3GPP) includes: a User Equipment (UE), an E-UTRAN NodeB (eNB) and an Evolved Packet Core (EPC). FIG. 1 is a diagram showing the structure of the LTE system, wherein an air interface (Uu) is adopted between the UE and the eNB, and an S1 interface is adopted between the eNB and the EPC.
In the LTE system, when the UE sends an attach request to the EPC, the EPC sets up a default PDN connection with the UE, i.e., the EPC sets up a default bearer, and allocates an IP address to the UE. Then, due to a service requirement, when the UE sends a request for setting up a second PDN connection to the EPC and the EPC receives the request, the EPC will set up a default bearer with the UE again and allocate a new IP address to the UE. The UE distinguishes different PDNs by Linked EPS Bearer Id (LBI) information.
When the UE does not need to keep connection with a certain PDN, it will send a PDN release request message to the EPC to indicate that the EPC needs to delete the PDN connection, wherein the release request includes releasing the default bearer and releasing all private bearer resources. When determining that the PDN requested to be released is not the last PDN with the UE, the EPC deletes all resources connected to the PDN, including the default bearer and all private bearers thereof. FIG. 2 is a process that a UE requests to release a certain PDN, and the process may include:
S201: The UE sends an uplink message to the EPC via an eNB, wherein the uplink message contains a PDN disconnection request, the PDN disconnection request contains the LBI information of the PDN requested to be released.
S202: After receiving the uplink message, the EPC sends a bearer release request to the eNB, wherein the bearer release request contains context request information of releasing an EPS (Evolved Packet System) bearer; and starts a T3495 timer.
S203: After receiving the bearer release request, the eNB sends a Radio Resource Control (RRC) connection reconfiguration message to the UE, wherein the RRC connection reconfiguration message contains the context request information of releasing the EPS bearer.
S204: The UE receives the RRC connection reconfiguration message, performs connection reconfiguration, and sends an RRC connection reconfiguration completion message to the eNB after the connection reconfiguration is completed.
S205: After receiving the RRC connection reconfiguration completion message, the eNB sends a bearer release response message to the EPC.
S206: The UE sends an uplink message to the EPC via the eNB, wherein the uplink message contains the context information of releasing the EPS bearer reception.
S207: When receiving an uplink message containing the context information of releasing the EPS bearer reception within the timing range of the T3495 timer, the EPC deletes all resources of the PDN connection, including the default bearer and all private bearers thereof.
Otherwise, besides the flow of releasing the PDN connection shown in FIG. 2, the EPC will also actively initiate a PDN connection release request because of insufficient resources. For example, in an idle mode, the EPC will locally release the PDN but does not notify the UE. When switching from an idle mode to a connection mode, the EPC will synchronize bearer resources by a Service Request message or Tracking Area Updating (TAU) flow. FIG. 3 is a process that an EPC actively initiates a PDN release request in a connection state, and the process may include:
S301: The EPC sends a bearer deleting request to the eNB, wherein the bearer deleting request contains the context request information of releasing the EPS bearer, and starts the T3495 timer.
S302: After receiving the bearer deleting request, the eNB sends an RRC connection reconfiguration message to the UE, wherein the RRC connection reconfiguration message contains the context request information of releasing the EPS bearer.
S303: The UE performs connection reconfiguration and then sends the RRC connection reconfiguration completion message to the eNB after the connection reconfiguration is completed.
S304: The eNB sends a bearer release response message to the EPC.
S305: The UE sends an uplink message to the EPC via the eNB, wherein the uplink message contains the context information of releasing the EPS bearer reception.
S306: After receiving the uplink message sent by the UE and containing the context information of releasing the EPS bearer reception within the timing range of the T3495 timer, the EPC deletes all resources of the PDN connection, including the default bearer and all private bearers thereof, and stops the T3495 timer.
The process of releasing the PDN bearer initiated by the EPC in the above mentioned process may be not notified to the UE due to the network status and the like. FIG. 4 is a process that the EPC cannot notify the UE due to the network status, and the process may include:
S401: The EPC sends a bearer deleting request to the UE via the eNB, wherein the bearer deleting request contains the context request information of releasing the EPS bearer, and then starts the T3495 timer.
S402: The UE detects the bearer deleting request, and when the detection is failed, for example, the detection is failed due to a security problem of the deleting request, the UE will discard the bearer deleting request.
S403: When failing to receive the response message of the UE within the timing range of the T3495 timer, the EPC sends the bearer deleting request to the UE again via the eNB, and starts the T3495 timer.
S404: The UE detects the bearer deleting request, and when the detection is failed, for example, the detection is failed due to a security problem of the deleting request, the UE will discard the bearer deleting request.
S405: When failing to receive the response message of the UE within the timing range of the T3495 timer, the EPC sends the bearer deleting request to the UE again via the eNB, and starts the T3495 timer again.
S406: If still failing to receive the response message of the UE within the timing range of the T3495 timer after predetermined times, such as 5 times, the EPC locally deletes the PDN connection.
Under such condition, the release information of the PDN bearer is not notified to the UE, but the EPC has deleted the PDN connection, so that the PDN information stored by the EPC and that stored by the UE are not consistent. In addition, when the EPC stores one PDN connection which is set up between the EPC and the UE, for example, a first PDN connection, and the UE has set up at least two PDN connections with the EPC, if the UE needs to delete the first PDN connection, the UE sends a request message of releasing the first PDN to the EPC via the eNB; after the EPC receives the request message of releasing the first PDN, it determines that only the first PDN connection is existed between the EPC and the UE, and will reply a PDN release rejection message to the UE, wherein the rejection message contains a rejection reason: the last PDN connection is not allowed to be released, i.e., the PDN connection is the only connection between the UE and the EPC and therefore not allowed to be released.
In the related art, the UE will discard the PDN release process after receiving the rejection message. Since the UE cannot delete the PDN, when the UE sends data by the PDN which has been deleted at the EPC side, data cannot be sent to a corresponding end, so that reliability of data transmission is affected. Besides, the UE and the eNB do not release the corresponding PDN, therefore the PDN resources cannot be used by other UEs, so as to cause the waste of corresponding PDN resources.